Diazonium compounds are widely used as sensitizers in presensitized printing materials, and the most commonly employed diazonium compounds are diazo resins, typically the condensation products of p-diazodiphenylamine and formaldehyde. In preparing a light-sensitive layer for use in a presensitized lithographic printing plate, the diazo resin may be used either alone as shown in U.S. Pat. No. 2,714,066 (i.e., in the absence of any binder) or in admixture with a binder as described in Japanese Patent Application (OPI) No. 30604/1975 (the term "OPI" as used herein means an unexamined published Japanese patent application) corresponding to U.S. Pat. 4,275,138. In most of the diazosensitized lithographic printing plates manufactured today, a diazonium compound is used together with a polymer binder in order to provide for more increased press life.
Light-sensitive layers which comprise a diazonium compound and a polymer binder may be divided into two types, depending on the type of developer used to remove (i.e., develop) the layer in unexposed areas: (1) the "alkali" developable type of light-sensitive layer which is treated with an aqueous alkali developer, and (2) the "solvent" developable type of light-sensitive layer which is treated with an organic solvent-based developer. The alkali-developer type is increasingly used today, principally for safety and environmental reasons due to problems relating to disposal of the fatigued developer. How well this type of light-sensitive layer performs is determined by the properties of the binder used. The binder may be rendered alkali-devlopable either by copolymerizing it with a carboxylic acid-containing monomer as described in Japanese Patent Application (OPI) No. 30604/1975, or by reacting the hydroxyl group in polyvinyl alcohol with a cyclic acid anhydride such as phthalic anhydride in order to introduce the carboxylic acid group into the polymer as shown in U.S. Pat. No. 2,861,058. Whichever method is used, the resulting polymer has only a low level of abrasion resistance because of its structure, and the presensitized plate using this type of carboxylic acid-containing polymer as a binder in the light-sensitive layer provides a lithographic printing plate having a short press life. When polyvinyl acetal is used as the binder, a sturdy film is formed having high abrasion resistance; however, the lithographic printing plate sensitized by a layer containing polyvinyl acetal as the binder is of the organic solvent-developable type described above and cannot be treated with aqueous alkali developers.
It has been proposed that the abrasion-resistant polyvinyl acetal is modified to make the compound alkali-soluble, and that the so-modified polyvinyl acetal is mixed with a diazonium compound. Light-sensitive compositions prepared by this method are described in British Pat. Nos. 1,370,316 and 1,600,871 and U.S. Pat. No. 3,847,614. The binder in the light-sensitive composition described in British Pat. No. 1,370,316 contains an arylsulfonyl urethane which is prepared from a sulfonyl isocyanate but this compound has the drawback that it is very difficult to synthesize the sulfonyl isocyanate industrially at economically feasible costs. The method described in U.S. Pat. No. 3,847,614 consists of copolymerizing vinyl acetate with a monomer containing a carboxylic acid and subsequently treating the copolymer such that it is saponified and converted to the acetal form. However, this method has the disadvantage that the saponified copolymer is not converted into the acetal form in a sufficient amount, causing unsatisfactory abrasion resistance of the light-sensitive composition. The light-sensitive composition described in British Pat. No. 1,600,871 suffers from the low sensitivity of diazosulfonate.